Incrementally Resolved Phase-Shift Conflicts In Layouts For Phase-Shifted Features

ABSTRACT

Phase shifting allows generating very narrow features in a printed features layer. Thus, forming a fabrication layout for a physical design layout having critical features typically includes providing a layout for shifters. Specifically, pairs of shifters can be placed to define critical features, wherein the pairs of shifters conform to predetermined design rules. After placement, phase information for the shifters associated with the set of critical features can be assigned. Complex designs can lead to phase-shift conflicts among shifters in the fabrication layout. An irresolvable conflict can be passed to the design process earlier than in a conventional processes, thereby saving valuable time in the fabrication process for printed circuits.

CLAIM OF PRIORITY

This application claims priority to U.S. patent application Ser. No.10/377,341-4038, filed Feb. 27, 2003 by Shao-Po Wu and Yao-Ting Wang andentitled “Incrementally Resolved Phase-Shift Conflicts In Layouts ForPhase-Shifted Features” which claims priority of U.S. patent applicationSer. No. 09/823,380-7393, filed Mar. 29, 2001 by Shao-Po Wu and Yao-TingWang, and entitled “Incrementally Resolved Phase-Shift Conflicts InLayouts For Phase-Shifted Features”, which in turn claims priority toU.S. Provisional Application Ser. No. 60/243,524, filed Oct. 25, 2000 byShao-Po Wu, Yao-Ting Wang, Kent Richardson, Christophe Pierrat, andMichael Sanie, and entitled “Incrementally Resolved Phase-ShiftConflicts In Layouts For Phase-Shifted Features”.

RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.09/823,146-8659, filed Mar. 29, 2001 by Yao-Ting Wang, Kent Richardson,Shao-Po Wu, Christophe Pierrat, and Michael Sanie, and entitled“Conflict Sensitive Compaction for Resolving Phase-Shift Conflicts inLayouts for Phase-Shifted Features”.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

This invention relates to the field of printed circuit manufacturing. Inparticular, this invention relates to inserting and assigning phases tophase shifters on masks used to fabricate integrated circuits.

2. Description of Related Art

Conventional integrated circuit (IC) fabrication involves many steps incommon with other processes that impose physical structures in a layeron a substrate, such as laying ink in patterns on a page, or layingchrome in patterns on a quartz substrate. Some of the important stepsviewed at a high level are depicted in FIG. 1.

In step 110, engineers use a functional computer aided design (CAD)process, to create a schematic design, such as a schematic circuitdesign consisting of individual devices coupled together to perform acertain function or set of functions. The schematic design 115 istranslated into a representation of the actual physical arrangement ofmaterials upon completion, called a design layout 125, with a physicalCAD process 120. If multiple layers are involved, as is typical for anIC, a design layout is produced for each layer, e.g., design layouts 125a, 125 b, etc. FIG. 2 shows a sample design layout. A fabrication CADprocess 130 produces one or more fabrication layouts 135, such as masksfor each design layout 125 a. The one or more fabrication layouts 135are then used by a substantiation process 140 to actually producephysical features in a layer, called here the printed features layer149.

One recent advance in optical lithography called phase shiftinggenerates features in the printed features layer 149 that are smallerthan the features on the mask 135 a projected onto the printed featureslayer 149. Such fine features are generated by the destructiveinterference of light in adjacent separated windows in the mask calledshifters. FIG. 3 shows two adjacent shifters, 310 and 312, in a mask300. The shifters 310 and 312 are light transmissive areas on the maskseparated by an opaque area 311 with a width of Wm 313 when projectedonto the printed features layer 149. The projection of Wm onto theprinted features layer 149 is limited by the resolution of the opticalprocess. However, if the light of a single wavelength passing throughone of the shifters, e.g. 310, is out of phase (by 180 degrees or □radians) with the light of the same wavelength passing through the othershifter, e.g. 312, then an interference pattern is set up on the printedfeatures layer 149 during the substantiation process 140. Thisinterference generates a printed feature 350 having a width Wp 353 thatis less than the width Wm 313 of the opaque area projected onto theprinted features layer 149. In other embodiments, the width 313 andwidth 353 are much closer and can be equal. In each case, the width 353of the printed feature is less than that produced by the same opticalsystem without phase shifting.

The use of phase shifting puts extra constraints on the fabricationlayouts 135, and hence on the design layout, e.g. 125 a. Theseconstraints are due to several factors. One factor already illustratedis the need for finding space on the mask, e.g., 135 a, for the twoshifters, 310 and 312, as well as for the opaque area 311 between them.This precludes the one mask from placing additional features on theprinted features layer 149 in the region covered by the projection ofthe two shifters 310 and 312 and the opaque area 311. Another factor isthat overlapping or adjacent shifters on a single mask, used, forexample, to generate neighboring phase-shifted features, generally donot have different phases. Adjacent shifters with different phases willproduce a spurious feature.

Currently, design layouts 125 may provide the space needed for placementof phase shifters through design rules, but shifters are actually placedand simultaneously assigned a phase in the conventional fabricationdesign steps, not shown, in attempts to produce the fabrication layouts.As complex circuits are designed, such as by combining many standardcells of previously designed sub-circuits, shifters of different phasesmay overlap or become adjacent in the layouts, leading to phase-shiftconflicts. It is generally recognized that resolving phase-shiftconflicts should be done globally, after the whole circuit is laid out,because swapping the phases of a pair of shifters to resolve oneconflict can generate a new conflict with another neighboring featurealready located in the design or one added later. The conventional ICdesign systems try to reassign phases of individual pairs to resolve theconflicts at the end of the design process when all the phase conflictsare apparent. For example, iN-Phase™ software from NUMERICALTECHNOLOGIES, INC.™ of San Jose, Calif., uses this conventionaltechnique.

For example, FIG. 4 shows a T-junction element 440 that is desirablyformed with narrow phase-shifted features 443, 442 and 444 as well aswith wide non-critical features 441 and 445. FIG. 4A shows a pair ofshifters 410 and 420 needed to form the vertical phase-shifted feature443 of element 440. FIG. 4A also shows another shifter 415 disposedopposite shifter 410 to form the left half 442 of the horizontalphase-shifted feature of element 440. Similarly, FIG. 4A also shows afourth shifter 425 disposed opposite shifter 420 to form the right half444 of the horizontal phase-shifted feature of element 440. Shifters 415and 425 are so close that they violate a design rule requiring at leasta minimum spacing X between adjacent shifters. That is, separation 427is less than X.

In the conventional fabrication CAD process, not shown, the shifters410, 420, 415 and 425 are placed as shown and assigned phases, but thephase-shift conflict is not addressed until all the elements of thedesign layout have been accounted for. Then the design rule is appliedin which shifters 415 and 425 are replaced by a single shifter 430.

However, there is no assignment of phase for shifter 430 that cansimultaneously be opposite to the phases assigned to shifters 410 and420, because shifters 410 and 420 are already opposite to each other.Thus such a design has a conflict that cannot be solved by changing thephases assigned to the shifters. Some re-arrangement of shifters orfeatures or both is needed. In this example, however, the feature 440from the physical design layout does not allow shifter 430 to be movedand does not allow another shifter to be inserted. Thus the fabricationCAD process 130 cannot move or change the shifters enough to resolve theconflict.

When a phase-shift conflict is irresolvable by the fabrication CADprocess 130, then the physical CAD process 120 is run again to move orreshape the features, such as those of element 440. Process flow with anirreconcilable phase-shift conflict is represented in FIG. 1, whichshows that fabrication layouts 135 are produced along the arrow marked“Succeed” if the fabrication CAD process 130 succeeds, but that controlreturns to the physical CAD process 120 along the arrow marked “Fail” ifthe fabrication CAD process 130 fails, such as if it fails to resolveall phase conflicts.

While suitable for many purposes, the conventional techniques have somedeficiencies. As designs, such as designs for IC circuits, become morecomplex, the time and effort involved in performing the physical CADprocess 120 and the fabrication CAD process 130 increase dramatically,consuming hours and days. By resolving phase-shift conflicts at the endof this process, circumstances that lead to irresolvable phase-shiftconflicts are not discovered until the end of these time consumingprocesses. The discovery of such irresolvable phase-shift conflictsinduces the design engineers to start over at the physical CAD process120. The processes 120 and 130 are repeated until final design layoutsand fabrication layouts without phase-shift conflicts are produced. Thisprocedure multiplies the number of days it takes a foundry to beginproducing IC chips. In a commercial marketplace where IC advancementsoccur daily, such delays can cause significant loss of market share andrevenue.

Techniques are needed to discover and resolve phase-shift conflictsearlier in the sequence of physical layout designing and fabricationlayout designing. Repeatedly assigning phases to the same shifters isundesirable in such techniques, however, because such repetitionindicates inefficient processing and wasted processing resources.

SUMMARY OF THE INVENTION

According to techniques of the present invention, phase-shift conflictsare detected incrementally at each node of a hierarchical design treefor a design layout. This incremental detection is made efficient byseparating the placement of shifters from assignment of phases and byusing relative phases instead of absolute phases at each node of thehierarchy.

The incremental detection of phase shift conflicts provides theadvantage of avoiding wasted time and effort in the fabrication designprocess. If the phase-shift conflict cannot be resolved for a particularnode, then continued design of a fabrication layout is stopped beforeany further resources are expended. Redesign of the physical layout isemployed before further fabrication layout design is fruitful. Bydetecting such an irresolvable conflict as soon as two or more units areplaced to be adjacent or overlapping at a node, many hours of subsequentfabrication layout time are saved.

Another advantage of detecting irresolvable phase-shift conflicts in thefirst hierarchical unit is that it has the potential to reduce theeffort required in the physical CAD process as well. For example, designlayout modifications can be concentrated at the node in the hierarchyexperiencing the irresolvable conflict, substantially simplifying andreducing the redesign process in many circumstances.

Another aspect of the invention is separating a phase assignment stepfrom the shifter placement step. The separation allows design rules tobe checked and placement to be corrected before any phases are assigned.This avoids wasting time and computational resources assigning phases toshifters that get merged, moved or eliminated due to the design rules.In addition, the separation allows the phase assignments to be appliediteratively, as sub-units are combined into units higher in the designhierarchy.

In another aspect of the invention, the shifters are associated withrelative phases rather than absolute phases. Using relative phases, foreach shifter pair, the two separate shifters adjacent to a singlephase-shifted feature are assigned a phase difference of 180 degrees.This provides the advantage of avoiding repeated changes to the absolutephase associated with a shifter. Relative phases provide sufficientinformation to detect conflicts without knowing which shifter actuallygets assigned which absolute phase. This also makes efficient thecombination of units into a unit higher in the hierarchy. The two unitscan be assigned relative phases (i.e., differences of 0 or 180 degrees)without changing the assignment of relative phases of the subunits orshifters within the units. Only after the whole circuit at the root nodeof the hierarchy has no phase-shift conflicts are the relative phasesconverted to absolute phases, starting at the two highest subunitsbranching from the root and working gradually back down the hierarchy tothe atomic “leaf” nodes of the hierarchy.

According to one aspect of the invention, techniques for providing alayout for shifters include establishing placement of multiple pairs ofshifters for a set of critical features. A critical feature employsphase shifting. The set of critical features constitutes a subset of allcritical features in a layout. After establishing placement of the pairsof shifters, phase information for the shifters associated with the setof critical features is assigned.

According to another aspect of the invention, techniques for providing alayout for shifters include identifying a first critical sub-unit of ahierarchical unit of a design layout. A critical sub-unit includes acritical feature that employs phase shifting, and includes placedshifters for the critical feature. Phase information for the firstcritical sub-unit is assigned prior to identifying a different criticalsub-unit of a different hierarchical unit.

According to another aspect of the invention, techniques for providing alayout for shifters include establishing placement of a pair of shiftersassociated with a critical feature. A critical feature employs phaseshifting. Relative phase information is assigned for the pair ofshifters.

According to another aspect of the invention, techniques for identifyingphase shift conflicts include establishing placement of shifters for aset of critical features. A critical feature employs phase shifting. Theset of critical features constitutes less than all critical features ina layout. After establishing placement of the shifters, and prior toestablishing placement for all shifters for all critical features in thelayout, it is determined whether there is a phase shift conflict among aset of shifters associated with the set of critical features.

According to another aspect of the invention, techniques for identifyingphase shift conflicts include identifying a first critical sub-unit of ahierarchical unit of a design layout. A critical sub-unit includes acritical feature that employs phase shifting. It is determined whetherthere is a phase shift conflict within the first critical sub-unitbefore determining whether there is a phase shift conflict among allsub-units within the unit.

In various aspects, the techniques are for a method, a computer-readablemedium, a system, a computer system, a fabrication layout, such as amask, and a device, such as a printed circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 is a block diagram showing the sequence of processes and layoutsutilized in the formation of printed features layers according to oneembodiment.

FIG. 2 is a plan view of an example design layout.

FIG. 3 is a plan view of two shifters on a mask and the resultingprinted feature on the printed features layer.

FIGS. 4A & 4B are plan views of example elements having features thatemploy shifters that lead to phase-shift conflicts.

FIG. 5 is a diagram of the hierarchical tree representation of thedesign layout in FIG. 2.

FIG. 6 is a flowchart illustrating the phase-shift conflict process atthe cell level according to one embodiment.

FIG. 7 is a flowchart illustrating the phase-shift conflict process at ahierarchical unit above the cell level according to an embodiment.

FIGS. 8A, 8B and 8C are flowcharts illustrating the steps for themodified design layout process according to embodiments.

FIGS. 9A, 9B and 9C show plan views of elements of a printed featureslayer adjusted according to the design layout process of embodiments.

FIG. 9D shows a phase assignment graph associated with a hierarchicalunit having a phase shift conflict.

FIG. 9E shows a phase assignment graph associated with the hierarchicalunit that resolves the phase conflict according to one embodiment.

FIG. 9F shows a phase assignment graph associated with the hierarchicalunit that resolves the phase conflict according to another embodiment.

FIG. 10 is a block diagram of a computer system according to oneembodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A method and apparatus for fabricating printed features layers, such asin integrated circuits, are described. In the following description, forthe purposes of explanation, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be apparent, however, to one skilled in the art that the presentinvention may be practiced without these specific details. In otherinstances, well-known structures and devices are shown in block diagramform in order to avoid unnecessarily obscuring the present invention.

Functional Overview

Techniques are provided for designing and fabricating printed featureslayers using a conflict sensitive compaction process 160 in the physicalCAD process 120, and a modified phase conflict process 150 in thefabrication CAD process 130, as shown in FIG. 1. In the remainder ofthis section the relationship between the two techniques is described ata high level. In the following sections the modified phase conflictprocess, using incremental resolution of phase conflicts, is describedin more detail. In subsequent sections, the conflict sensitivecompaction techniques are described in more detail.

The conflict sensitive compaction process 160 uses information suppliedby the fabrication CAD process 130 about the existence of one or moreparticular phase-shift conflicts in order to adjust the arrangement ofelements and features in one or more design layouts 125.

The modified phase conflict process 150 separates the task of placingshifters, for example with a placement engine, from the task ofassigning phases to those shifters. In particular, relative phases areassigned to shifters on a hierarchical unit basis, using a coloringengine. Coloring means assigning phase information to units, such asrelative phases for pairs of shifters. With the relative phases soassigned, the modified phase conflict process 150 determines whetherthere is a phase-shift conflict within the unit. Absolute phases are notassigned until relative phases without phase-shift conflicts can beassigned to each unit in the hierarchy of the design layout.

If any unit has a phase-shift conflict that cannot be resolved bychanging shifters or the relative phase assignments, then the modifiedphase conflict process 150 notifies the physical CAD process 120 of thephase-shift conflict and provides information about the particularphase-shift conflict. The fabrication design process does not proceedwith subsequent units in the hierarchy. In this way, phase-shiftconflicts are found and resolved incrementally, before time andcomputational resources are expended attempting to place shifters andassign phases to them for all the phase-shifted features in the entiredesign layout.

Hierarchical Layouts

A hierarchy can represent a layout. For example, as shown in FIG. 2, thecircuit design layout 290 comprises a final cell, or hierarchical unit,A 200, which comprises sub-units B 220, C 240, and D 260 which arethemselves parent cells for the units disposed in them. For example,parent cell C 240 comprises identical cells G1 241, G2 242, G3 243, G4244, G5 245 and G6 246, and parent cell F1 224 comprises leaf cells L1233 and M1 234 which comprise the primitive geometric structuresillustrated in FIG. 2A. Parent cell E1 222 includes leaf cells J1 231and K1 232; and parent cell E2 228 includes leaf cells J2 237 and K2238. Parent cell F2 226 includes leaf cells L2 235 and M2 236.

The hierarchical tree layout 599, shown in FIG. 5, illustrates thedescribed cells in a tree format with the leaf cells at the bottom oftree and with the final cell A 200 at the top of the tree. Each of theleaf cells is also sometimes referred to as the leaf node or a childcell, while each of the cells above the leaf nodes is sometimes referredto as a parent cell or simply a node. Any node can also be called ahierarchical unit of the design. The integrated circuit design layout200 of FIG. 2 is provided simply to demonstrate the hierarchical natureof design layouts in general, and for integrated circuits in particular.

The items on a mask can also be represented as hierarchical units,according to a related pending U.S. patent application Ser. No.09/154,397 entitled “Method and Apparatus for Data Hierarchy maintenancein a System for Mask Description,” filed on Sep. 16, 1998, invented byFang-Cheng Chang, Yao-Ting Wang and Yagyensh C. Pati.

Modified Phase Conflict Process

The modified phase conflict process 150 operates incrementally onhierarchical units of the design layout. The described embodiment beginswith a leaf cell and proceeds up the hierarchy to the root cell, but theprocess 150 can begin with any unit below the root cell. For example, ifthe design layout's hierarchy is represented by the tree in FIG. 5, themodified phase conflict process 150 of the described embodiment wouldfirst operate on one of the leaf cells, i.e., 231, 232, 233, 234, 235,236, 237, 238, or 241, 242, 243, 244, 245, 246, or 262, 264, 266. Theselection of the first leaf cell, and the progression through other leafcells, can be performed in any way known in the art. If the first leafcell is J1 231, the described embodiment would select as the next unitanother leaf cell, e.g., K1 232, which is combined with J1 231 by thenext higher node in the hierarchy, i.e., E1 222. After these units areprocessed, the described embodiment would process unit E1 222. However,before processing unit B 220, the described embodiment first processesthe other units, or nodes, combined by unit B 220, i.e., F1 224, F2 226,and E2 228. Since each of these units have subunits, their subunitsshould be processed before the respective units. Thus, in the describedembodiment, leaf cells L1 233 and M1 234 are processed in turn beforeprocessing unit F1 224.

In another embodiment, the first node processed on a branch may be anynode in the hierarchy 599 below the root node A 200. However, if thefirst node selected is not a leaf cell, all the subunits in the firstnode are processed together. One or more other nodes are first processedon respective other branches in the tree. In the following discussion,the first node selected on any branch for processing is called a cell.For example, if B 220 is the first node processed on its branch, thenall the nodes below B 220, i.e., 220-238, are included in a cell. Othercells are needed, in this example, for the remaining branches to nodes Cand D and below. For example, node C may be processed first in itsbranch, making the nodes 240-246 one cell. In a contrasting example, thebranch involving node D 260, first processes the leaf nodes, 262, 264and 266, making those the cells on their branches.

In one embodiment of the invention, shifters are initially placed in acell, and in subsequent hierarchical units the shifters are corrected orassigned relative phases or both, but are not initially placed.

FIG. 6 is a flowchart illustrating the phase-shift conflict process 150at the cell level, according to one embodiment. This process can beexecuted in a computer system, such as the computer system shown in FIG.10. In step 605 the process makes the next cell of the cells in thehierarchy the current cell for processing. In step 610 the processidentifies shifted features in the current cell of the design layout. Instep 620, the process places shifters in pairs, the shifter havingshapes and positions related to the positions and shapes of thephase-shifted features in the current cell in ways known in the art. Instep 630, the process performs design rules checking and correction(DRC&C) for the shifters of the current cell. For example, if step 620placed shifters as shown in FIG. 4A, then the design rule that forbidsspacing between two adjacent shifters from being smaller than a certaindistance X would force the DRC&C step 630 to combine shifters 415 and425 and derive a single shifter 430. The shifter 430 is derived from theinitial shifters 415 and 425. In a trivial element of this process,shifters 410 and 420 are derived to be the same as their initialplacement. In step 640, the process assigns relative phases to theshifters in the current cell—this is called intra-cell coloring. Usingrelative phases, for each shifter pair, the two separate shiftersadjacent to a single phase-shifted feature are assigned a phasedifference of 180 degrees. This step can be accomplished using waysknown in the art, such as the standard graph traversal algorithm. In thegraph-traversal algorithm, a phase-assignment graph is constructed inwhich each given shifter is a node and adjacent shifters that constrainthe phase of the given shifter are represented by links. Two kinds oflinks are represented, an opposite phase link and a same phase link. Anopposite phase link is indicated to form a critical, phase-shiftedfeature. A same phase link is employed when two shifters (nodes) areclose together without an intervening critical phase-shifted feature.The links represent the relative phases without fixing an absolutephase. An example of a phase-assignment graph is given in more detail ina later section.

Unlike conventional fabrication layout design, this embodiment separatesthe placement of shifters in step 620, as performed by a placementengine, for example, from the assignment of phases to the shifters instep 640, as performed by a coloring engine providing relative phasesfor the shifters, for example. By assigning relative phases in step 640,rather than absolute phases, this embodiment does not fix the absolutephase of the shifters; but, instead, allows the relative phases to beswitched as needed to resolved future phase conflicts before fixing theabsolute phases of the shifters in this cell. This process makes it easyto swap the phases of the necessary shifter pairs in the cell with asingle command or notation, if that turns out to be needed to resolvesome future phase-shift conflict.

In step 650, the relative phases are used to determine whether there isa phase-shift conflict in the current cell. For example, FIG. 4Aillustrates a feature 440 that leads to a phase-shift conflict asrepresented by FIG. 4B. This phase-shift conflict can be detected withthe relative phases assigned to the shifters. Another common phase shiftconflict arises with odd cycle shifters—a set of shifters in which anodd number of shifters are associated with closely spaced phase-shiftedfeatures.

Unlike conventional fabrication layout design processes, this embodimentdetects a phase-shift conflict at the cell level, rather than after allshifters have been placed and assigned absolute phases for the wholedesign layout. Consequently, a phase-shift conflict resolution can beattempted at the level of the current cell, which is a simpler problemthan resolving phase-shift conflicts for the entire design layout.

If no phase-shift conflict remains in the current cell, then controlpasses to step 670 in which the current cell is added to a pool ofsuccessfully colored hierarchical units. Units are successfully coloredif relative phases can be assigned that do not cause phase-shiftconflicts. The colored unit pool may be maintained in memory or onpermanent storage device accessible to the fabrication layout designprocess 130. In step 680 of this embodiment, it is determined whetherall the cells for the next higher node of the hierarchy are available inthe colored unit pool. If they are, then processing can begin for thenext higher node in the hierarchy. If all the cells needed by the nexthigher node in the hierarchy are not already in the colored unit pool,then another cell needed by the next higher node is made the currentcell in step 605.

If it is determined in step 650 that there is a phase-shift conflict inthe current cell, then control passes to step 660, which attempts toresolve the conflict for the current cell within the fabrication layoutdesign process 130. It is assumed in this embodiment that thefabrication layout design process 130 can change the position or shapeof shifters, consistent with the shifter design rules, and can changethe relative or absolute phases of the shifters, but cannot change theposition or shape of features that appear in the design layout 125 for aprinted features layer 149. Step 660 includes any methods known in theart to resolve phase-shift conflicts within the fabrication layoutdesign process. Known methods include replacing an offending shifterwith a stored shifter that is differently positioned or shaped, breakingup odd cycle shifters by replacing one of the shifters in thecombination with two separated shifters, and obtaining manual input froman operator to re-shape or re-position or break-up a shifter or toprovide relative phase information for a shifter. Another method is toallow two opposite-phase shifters to produce a spurious feature, andthen to expose the spurious feature in a different stage of thefabrication process to cause the removal of the spurious feature. Thetwo opposite-phase shifters result either from splitting one shifter intwo, or allowing two shifters to be positioned closer than a design rulelimit without joining the two shifters.

Another method to resolve phase-shift conflicts within a hierarchicalunit involves introducing one or more new variants of a standard cell inthe hierarchical unit. Each variant has one or more pairs of shiftersreversed from their phases in the standard cell. This method involvesreplacing a standard cell with one of its variants in the hierarchicalunit.

If step 660 is able to modify the shifter layout for the cell, controlpasses to step 620 to place the shifters in the case in which a shiftershape has been changed. If step 660 also specified positions for theshifters, control returns to step 630 to perform DRC&C for the cell. Ifstep 660 also overrules DRC&C, control will pass back to step 640 toassign relative phases. The new arrangement of shifters and phases ischecked for phase-shift conflicts in step 650.

If step 660 is unable to provide different shifter shapes or positions,or if repeated changes to shifter shapes and positions do not remove allphase-shift conflicts in the current cell, then step 660 is unable toresolve the phase-shift conflict for the current cell, and step 660fails. Upon failure of step 660 to resolve one or more phase-shiftconflicts in the current cell, control passes to a point in the physicaldesign process 120 represented by transfer point 800 in FIG. 6. Thephysical design process 120 then rearranges features in the designlayout 125.

FIG. 7 is a flowchart illustrating the phase-shift conflict process 150at a general hierarchical unit level, according to one embodiment of thepresent invention. At step 705 the process makes the next higher nodethe current unit, such as when all the cells within a parent node havebeen processed. If the general hierarchical unit is a cell first beingprocessed, then step 705 can be skipped. In step 720 the processidentifies subunits with phase-shifted features in the design layout ofthe current unit. If the current unit is the first cell being processedon its branch, then shifters have to be placed for the phase-shiftedfeatures, as shown in FIG. 6. However, if the current unit is made up ofsubunits that have already been processed, then the shifters for thephase-shifted features have already been initially placed.

In step 730, DRC&C is performed on the shifters for the current unit.During this step a shifter smaller than the allowed minimum width, or aspacing between two shifters that is smaller than the allowed minimumspacing X, will be discovered and corrected, for example.

In step 740, the shifters in all the subunits in the current unit willbe assigned relative phases, not by reassigning the relative phase ofall shifters in the unit, but by adjusting the relative phase betweensubunits, e.g., by recording that a first subunit is 180 degrees out ofphase from a second sub-unit—this is called inter-cell coloring. In oneembodiment, inter-cell coloring is accomplished by simply reversing thepolarity of the needed relative phases of a subunit. This preserves therelative phases of all the shifters within the subunit. In anotherembodiment this is accomplished by adding a link between nearby shiftersin the phase-assignment graph for this current hierarchical unit.

Unlike conventional fabrication layout design processes, this embodimentprovides relative phase information separately from positioning theshifters. Moreover, this embodiment provides a way of incrementallybuilding up the relative phase information from lower hierarchical unitlevels all the way to the top level. Again, as above, by assigningrelative phases in step 740, rather than absolute phases, thisembodiment does not set the absolute phase of the shifters; but,instead, allows the relative phases to be switched as needed to resolvefuture phase conflicts in higher units in the hierarchy before fixingthe absolute phases of the shifters in this unit. This embodiment makesit easy to swap the phases of all the shifters in the unit with a singlecommand or notation, if it turns out to be needed to resolve some futurephase-shift conflict at a unit higher in the hierarchy of the designlayout.

In step 750, the relative phases are used to determine whether there isa phase-shift conflict in the current unit. Unlike conventionalfabrication layout design processes, this embodiment detects aphase-shift conflict at the unit level, rather than after all shiftershave been placed and assigned absolute phases for the whole designlayout. Consequently, a shift conflict can be detected early. Inaddition the phase-shift conflict resolution can be attempted at thelevel of the current unit, which is a simpler problem than resolvingphase-shift conflicts for the entire design layout.

If it is determined in step 750 that there is a phase-shift conflict inthe current unit, then control passes to step 760, which attempts toresolve the conflict for the current unit within the fabrication layoutdesign process 130. As in step 660 above, step 760 is not limited to anyparticular technique for resolving phase-shift conflicts within afabrication layout design process. If step 760 is able to modify theshifter layout for the unit, control passes to step 730 to perform DRC&Cfor the unit. If step 760 involves a method that overrules a design ruleusually applied during DRC&C, control will pass back to step 740 toassign relative phases. The new arrangement of shifters and phases isthen checked for phase-shift conflicts in step 750.

If the methods applied in step 750 are unable to provide differentshifter shapes or positions, or if repeated changes to shifter shapesand positions do not remove phase-shift conflicts in the current unit,then step 760 fails. Upon failure of the methods applied in step 760 toresolve phase-shift conflicts in the current unit, control passes to apoint in the physical design process 120 represented by transfer point800 in FIG. 7. The physical design process 120 then rearranges featuresin the design layout 125, if possible and permitted.

If no phase-shift conflict remains in the current unit, then controlpasses to step 755. If the current unit is the root unit of thehierarchy, then the fabrication layout design is complete and withoutphase-shift conflicts; thus the fabrication design process 130 hassuccessfully produced fabrication layout 135. Step 755 determineswhether the current unit is a root unit of the hierarchy. If it isdetermined in step 755 that the current unit is the root unit, thencontrol passes to step 790. In step 790, absolute phases are associatedwith the relative phases assigned to each shifter in the fabricationlayout 135, the fabrication layout 135 is stored, and the fabricationdesign process ends successfully at point 795.

If the current unit is not the root unit of the hierarchy, then controlpasses to step 770 in which the current unit is added to the pool ofsuccessfully colored units. Control then passes to step 780, in which itis determined whether all units for the next higher node in thehierarchy are already in the colored unit pool. If all units for thenext higher node are already in the colored unit pool, then the nexthigher node is made the current unit, by passing control to step 705. Ifall units for the next higher node are not in the colored unit pool,then another node needed by the next higher node is made the currentunit, in step 785.

In this way, hierarchical units with relative phases assigned, and withno phase conflicts, are accumulated in the colored units pool. The unitsin this pool represent resources that can be readily re-used in otherdesigns, because they are known to be free of internal phase-shiftconflicts.

Conflict Sensitive Compaction

The physical design process 120 is modified to include conflictsensitive compaction 160 in an embodiment of the invention. FIG. 8A is aflowchart illustrating steps for a modified design layout processaccording to one embodiment of the invention. In this embodiment,control passes to transfer point 800 when the fabrication design processis unable to resolve a phase-shift conflict. In step 810, the processidentifies particular features with unresolved phase-shift conflictsbased on information received from the fabrication design process 130.If a conventional fabrication design process were employed, thisinformation first becomes available only for the entire design layout.However, in this embodiment, the information about a phase-shiftconflict becomes available for the first hierarchical unit thatencounters an irresolvable phase-shift conflict. Herein, an irresolvablephase-shift conflict indicates a phase-shift conflict that could not beresolved by the fabrication design process. In the described embodiment,the information includes identification of the hierarchical unit inwhich the irresolvable phase-shift conflict was found. In anotherembodiment, the information includes the amount of space needed toresolve the conflict with additional shifters. In another embodiment,the information includes a list of features linked by a loop in aphase-assignment graph with the feature having the phase-shift conflict.

In step 820, the process adjusts the design layout based on theinformation provided about the particular phase-shift conflicts, andproduces an adjusted design layout, 125 b. In one embodiment, theadjustment is confined to the features within the same hierarchical unitthat encountered the irresolvable phase-shift conflict. In analternative embodiment, the adjustment is confined to selected featureswithin a given distance of the particular features identified as havingunresolved phase-shift conflicts. The particular feature is includedamong the selected features. Unlike the conventional design process,which addresses phase-shift conflicts throughout the entire designlayout, these embodiments employ the design process 120 to solve a muchsmaller problem, one confined to a single unit in the hierarchy of thedesign layout, or one confined to a given distance from the particularfeatures identified with the phase conflict, or one confided to a subsetof features logically related by a loop in a graphical representation ofrelationships among shifters.

Different procedures can be used to adjust features in the hierarchicalor spatial vicinity of the phase-shift conflict. In one embodiment, thedesign layout in the vicinity is computed using the original designrules that produced the original design layout, such as the originalprocess-specific design rules, if several viable layouts are produced bythose design rules. In this case, it is suggested that a differentviable layout be used than was used to produce the original layout.However, if this method is used, there is no significantly improvedlikelihood that the new design will avoid a phase conflict. In someembodiments, such as where several viable solutions occur, multiplepotential solutions to a phase conflict are generated based on thelogically associated features. For example, a different one of theassociated features can be fixed in position for each differentpotential solution or set of potential solutions. The potentialsolutions are evaluated to produce a set of one or more values persolution. For example, the set of values includes the total area of thedesign associated with the potential solution design in one embodiment.In other embodiments, the set of values includes the number of featuresto move and the number of phase shift conflicts remaining. The potentialsolution providing a most favorable set of values is picked. For examplethe potential solution associated with the smallest area or fewestfeatures moved or fewest remaining conflicts is picked.

If another viable solution is tried, one embodiment adds step 830 toplace and color shifters according to the adjusted layout, and thencheck for phase-shift conflicts in the adjusted layout. If phase-shiftconflicts are still found in the adjusted layout, then another layout isselected from the viable layouts provided by the original design rules.The process continues until a viable layout is found which does notproduce a phase-shift conflict, or until the supply of viable options isexhausted.

FIG. 8B shows the steps that are used in an alternative embodiment ofstep 820, designated step 820 a, to adjust selected features within thevicinity of phase-shift conflicts.

In step 840, a critical feature among the selected features is madenon-critical. Herein a critical feature is one that employs phaseshifting; thus a non-critical feature is one that does not employ phaseshifting. The ability of an adjustment making a critical featurenon-critical to remove phase-shift conflicts is illustrated in FIGS. 9Aand 9B.

FIG. 9A shows an element 940 with five critical features 941, 942, 943,944 and 945. Shifters 910 and 920 have opposite phases to form criticalfeature 943. This element leads to a phase-shift conflict becauseshifter 930 cannot simultaneously have opposite phase from both shifters910 and 920. This phase-shift conflict was not resolvable by thefabrication layout design process because there was no room to insertanother shifter or split shifter 930. According to this embodiment,feature 943 can be made non-critical. In this case, illustrated in FIG.9B, non-critical feature 953 replaces critical feature 943 in element950. As a consequence, shifters 910 and 920 can be replaced by shifters914 and 924 spaced farther apart. In addition, there is no longer aninducement for shifters 914 and 924 to have opposite phase. When placedand colored in the fabrication design process, shifters 914 and 924 maybe given the same phase, and shifter 930 may assume an opposite phase toboth, thus resolving the phase-shift conflict.

It is appropriate to have new design rules that demand more space forplacing features if such design rules are applied only in the context ofphase-shift conflicts, because the benefit of removing the phase-shiftconflict is considered worth the expenditure of extra layout area.Sample new design rules include placing edges farther apart on featuresin the vicinity of an irresolvable phase-shift conflict, and placingcritical features father apart in the vicinity of an irresolvablephase-shift conflict. In step 850, new design rules applicable inphase-shift conflict situations are applied to critical features amongthe selected features. In step 860, other new design rules applicable inphase-shift conflict situations are applied to non-critical featuresamong the selected features. Steps 850 and 860 are separate to allow thenew phase-shift conflict design rules to be different for criticalfeatures and for non-critical features.

FIG. 9C illustrates how new design rules for critical features in thevicinity of a phase-shift conflict can resolve a phase-shift conflict.In this case, the phase-shift conflict caused by the element 940 in FIG.9A, is communicated by the fabrication design process 130 to anembodiment of process 160 a that includes step 850. Based on theinformation about the phase-shift conflict, in step 850, the processapplies a new design rule calling for greater separation betweencritical features than called for in the original design rules. Thiscauses features 945 and 944 to be moved further away from features 941,942 and 943 in the adjusted design layout, as shown in FIG. 9C. Withthis arrangement, shifter 930 can be replaced by two separate shifters932 and 934, which are far enough apart to have opposite phases fromeach other. With the extra space in this arrangement of shifters, thecoloring engine can assign shifters 910 and 934 a first phase, andassign shifters 932 and 922 the opposite phase. Then phase shiftedfeature 943 can be produced by the opposite phases of shifters 910 and922. Simultaneously, phase-shifted features 942 and 941 can be producedby the opposite phases of shifters 910 and 932; while phase-shiftedfeatures 944 and 945 can be produced by the opposite phases of shifters922 and 934. The resulting element 940 a includes a non-critical feature948 between the critical features 942 and 944 in the gap caused byseparating shifters 932 and 934.

A characteristic of the new design rules is the expected increase inlayout area associated with the adjusted layout compared to the originallayout. For example, the layout area associated with FIG. 9C is greaterthan layout area associated with FIG. 9A. It is possible that thephysical layout design process can compensate for this increased area bythe rearrangement of other features so that the total area for a cell orhierarchical unit of the design layout is not increased. In a sense, thecell or unit is re-compacted to accumulate space in the vicinity of thefeatures associated with an irresolvable phase conflict. Thisaccumulation of space or increase in layout area or both is hereintermed reverse compaction.

FIG. 8C is a flowchart illustrating steps for a modified design layoutprocess according to another embodiment of the invention. As in FIG. 8A,control passes to transfer point 800 when the fabrication design processis unable to resolve a phase-shift conflict. In step 810, the processidentifies particular features with unresolved phase-shift conflictsbased on information received from the fabrication design process 130.In this embodiment, the information includes a list of features in thesame graphical loop of a phase-assignment graph.

In step 820 b, the process adjusts the design layout based on theinformation provided about the particular phase-shift conflicts, andproduces an adjusted design layout, 125 b. In this embodiment, theadjustment is confined to features in the same graphical loop of relatedshifters, regardless of whether these features are neighbors or whetherthe features are within a specified distance of the irresolvablephase-shift conflict, or even whether they are in the same hierarchicalsubunit. In the described embodiment, the loop includes shifters in thesame hierarchical subunit. The particular feature is included among theselected features. If this method is used, there is a significantlyimproved likelihood that the new design will avoid a phase conflict. Ifa critical feature is moved, however, there is a chance that a shifteris placed close to another shifter that can lead to a phase-shiftconflict. Therefore, another embodiment using this method also adds step830 to place and color shifters according to the adjusted layout, andthen check for phase-shift conflicts in the adjusted layout. Ifphase-shift conflicts are still found in the adjusted layout, thenanother of the selected features is made modified. The process continuesuntil a modification is found which does not produce a phase-shiftconflict, or until the list of features on the same graphical loop isexhausted. The steps to adjust selected features shown in FIG. 8B forstep 820 a may also be used in step 820 b. An example of this embodimentis illustrated with respect to FIGS. 9D, E and 9F.

FIG. 9D shows nine critical features 985 and a correspondingphase-assignment graph. The phase assignment graph is made up of nodes980 representing shifters and links. In this example, each link 982 isan opposite phase link, connecting shifters that have opposite phases toproduce the nine critical features. For example, link 982 a indicatesthat the shifter at node 980 a and the shifter at node 980 b haveopposite phases to form the critical feature 985 a. This phaseassignment graph is an example of an odd-cycle graphical loop thatconstitutes a detectable phase-conflict. To illustrate the conflict,assume that the shifter at node 980 a is given a first phase value(either 0 or □). Then the shifters at nodes 980 b and 980 i have thesecond phase value, and the shifters at nodes 980 c and 980 h have thefirst phase value, and the shifters at nodes 980 d and 980 g have thesecond phase value, and the shifters at nodes 980 e and 980 f have thefirst phase value. This leads to a phase-shift conflict at 985 e,because opposite phases are needed in the shifters at nodes 980 e and980 f to form the critical feature 985 e, yet the shifters at nodes 980e and 980 f have the same phase. It is assumed that this phase-shiftconflict was not resolvable by the fabrication layout design processbecause there was no room to insert another shifter or split shifters ateither node 980 e or 980 f.

According to this embodiment, any feature formed by the shifters on thegraphical loop of FIG. 9D may be moved or made non-critical to resolvethis conflict. It is not necessary that that the adjusted feature bewithin a certain distance of the feature having the conflict. Forexample, it is not necessary that the adjusted feature be within circle987 centered on critical feature 985 e. It is also not necessary thatthe adjusted feature be within the same hierarchical subunit of thefeature having the conflict. For example, the graphical loop of FIG. 9Dmay span several hierarchical subunits, such as parent cells E1, F1, E2and F2 of FIG. 2.

For example, as illustrated in FIG. 9E, non-critical feature 985 xreplaces critical feature 985 h. As a consequence, shifters at nodes 980i and 980 h can have the same phase value, resolving the phase shiftconflict on the loop. Shifter having the same phase are indicated by adifferent link 984, indicated in FIG. 9E by the thin line segment.Effectively, shifters at nodes 980 i and 980 h can be combined, reducingthe number of shifters to 8 and eliminating the odd-cycle graphicalloop. If the shifters do not form a critical feature and are far enoughapart, no link at all needs to connect them and each is free to assumeany value. If this were the case, no link would connect nodes 980 h and980 i. Note that the feature adjusted is neither in the samehierarchical subunit nor within the circle 987 centered on theparticular feature 985 e originally identified as having the unresolvedphase-shift conflict.

FIG. 9F illustrates movement of an adjusted feature can resolve aphase-shift conflict. In this case, critical feature 985 h in FIG. 9D isreplaced by critical feature 985 y in FIG. 9F. Effectively, criticalfeature 985 h is moved to the position of critical feature 985 y. It isassumed that critical feature 985 h is moved because there is more roomin its neighborhood than in the neighborhood of the other featuresconnected by the graphical loop. Alternatively, it is moved because itis easier to accumulate space around it during reverse compaction. Toform critical feature 985 y, a new shifter is placed at node 980 y,adding a tenth shifter to the graph, and is linked with an oppositephase link 982 y. Since no critical feature is positioned between theshifters at nodes 980 h and 980 i, these shifters can have the samephase value. If the shifters are far enough apart, no link at all needsto connect them and each is free to assume any value. If this were thecase, no link would connect nodes 980 h and 980 i. In either case, thephase-shift conflict on the loop is resolved. Note that the featureadjusted is neither in the same hierarchical subunit nor within thecircle 987 centered on the particular feature 985 e originallyidentified as having the unresolved phase-shift conflict.

The conflict sensitive compaction process depicted in FIG. 1 includesany adjustment in layout based on phase-shift conflict information, suchas reverse compaction and the selection of alternative viable layouts,whether the adjustment be on the level of the entire design layout or onthe level of any hierarchical subunit of it.

In one embodiment, electrical constraints are also checked during thedesign adjustment process through the use of a layout modification tool.An example of a layout modification tool that checks electricalconstraints is the abraCAD™ tool, available from CADABRA DESIGNSYSTEMS™, a NUMERICAL TECHNOLOGIES™ company.

In the described embodiment, the modified phase conflict process 150,and the conflict sensitive compaction process 160, are implemented on acomputer system with one or more processors. User input is employed insome embodiments.

Hardware Overview

FIG. 10 is a block diagram that illustrates a computer system 1000 uponwhich an embodiment of the invention is implemented. Computer system1000 includes a bus 1002 or other communication mechanism forcommunicating information, and a processor 1004 of one or moreprocessors coupled with bus 1002 for processing information. Computersystem 1000 also includes a main memory 1006, such as a random accessmemory (RAM) or other dynamic storage device, coupled to bus 1002 forstoring information and instructions to be executed by processor 1004.Main memory 1006 also may be used for storing temporary variables orother intermediate information during execution of instructions to beexecuted by processor 1004. Computer system 1000 further includes a readonly memory (ROM) 1008 or other static storage device coupled to bus1002 for storing static information and instructions for processor 1004.A storage device 1010, such as a magnetic disk or optical disk, isprovided and coupled to bus 1002 for storing information andinstructions.

Computer system 1000 may be coupled via bus 1002 to a display 1012, suchas a cathode ray tube (CRT), for displaying information to a computeruser. An input device 1014, including alphanumeric and other keys, iscoupled to bus 1002 for communicating information and command selectionsto processor 1004. Another type of user input device is cursor control1016, such as a mouse, a trackball, or cursor direction keys forcommunicating direction information and command selections to processor1004 and for controlling cursor movement on display 1012. This inputdevice typically has two degrees of freedom in two axes, a first axis(e.g., x) and a second axis (e.g., y), that allows the device to specifypositions in a plane.

The invention is related to the use of computer system 1000 forproducing design layouts and fabrication layouts According to oneembodiment of the invention, layouts are provided by computer system1000 based on processor 1004 executing one or more sequences of one ormore instructions contained in main memory 1006. For example, themodified phase conflict process runs as a thread 1052 on processor 1004based on modified phase conflict process instructions 1051 stored inmain memory 1006. Such instructions may be read into main memory 1006from another computer-readable medium, such as storage device 1010.Execution of the sequences of instructions contained in main memory 1006causes processor 1004 to perform the process steps described herein. Inalternative embodiments, hard-wired circuitry may be used in place of orin combination with software instructions to implement the invention.Thus, embodiments of the invention are not limited to any specificcombination of hardware circuitry and software.

The term “computer-readable medium” as used herein refers to any mediumthat participates in providing instructions to processor 1004 forexecution. Such a medium may take many forms, including but not limitedto, non-volatile media, volatile media, and transmission media.Non-volatile media includes, for example, optical or magnetic disks,such as storage device 1010. Volatile media includes dynamic memory,such as main memory 1006. Transmission media includes coaxial cables,copper wire and fiber optics, including the wires that comprise bus1002. Transmission media can also take the form of acoustic or lightwaves, such as those generated during radio-wave and infra-red datacommunications.

Common forms of computer-readable media include, for example, a floppydisk, a flexible disk, hard disk, magnetic tape, or any other magneticmedium, a CD-ROM, any other optical medium, punchcards, papertape, anyother physical medium with patterns of holes, a RAM, a PROM, and EPROM,a FLASH-EPROM, any other memory chip or cartridge, a carrier wave asdescribed hereinafter, or any other medium from which a computer canread.

Various forms of computer readable media may be involved in carrying oneor more sequences of one or more instructions to processor 1004 forexecution. For example, the instructions may initially be carried on amagnetic disk of a remote computer. The remote computer can load theinstructions into its dynamic memory and send the instructions over atelephone line using a modem. A modem local to computer system 1000 canreceive the data on the telephone line and use an infra-red transmitterto convert the data to an infra-red signal. An infra-red detector canreceive the data carried in the infra-red signal and appropriatecircuitry can place the data on bus 1002. Bus 1002 carries the data tomain memory 1006, from which processor 1004 retrieves and executes theinstructions. The instructions received by main memory 1006 mayoptionally be stored on storage device 1010 either before or afterexecution by processor 1004.

Computer system 1000 also includes a communication interface 1018coupled to bus 1002. Communication interface 1018 provides a two-waydata communication coupling to a network link 1020 that is connected toa local network 1022. For example, communication interface 1018 may bean integrated services digital network (ISDN) card or a modem to providea data communication connection to a corresponding type of telephoneline. As another example, communication interface 1018 may be a localarea network (LAN) card to provide a data communication connection to acompatible LAN. Wireless links may also be implemented. In any suchimplementation, communication interface 1018 sends and receiveselectrical, electromagnetic or optical signals that carry digital datastreams representing various types of information.

Network link 1020 typically provides data communication through one ormore networks to other data devices. For example, network link 1020 mayprovide a connection through local network 1022 to a host computer 1024.Local network 1022 uses electrical, electromagnetic or optical signalsthat carry digital data streams. The signals through the variousnetworks and the signals on network link 1020 and through communicationinterface 1018, which carry the digital data to and from computer system1000, are exemplary forms of carrier waves transporting the information.

Computer system 1000 can send messages and receive data, includingprogram code, through the network(s), network link 1020 andcommunication interface 1018.

The received code may be executed by processor 1004 as it is received,and/or stored in storage device 1010, or other non-volatile storage forlater execution. In this manner, computer system 1000 may obtainapplication code in the form of a carrier wave.

In the foregoing specification, the invention has been described withreference to specific embodiments thereof. It will, however, be evidentthat various modifications and changes may be made thereto withoutdeparting from the broader spirit and scope of the invention. Thespecification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense.

1. A device having a plurality of critical features formed usingshifters, the shifters providing phase shifting, the device producedusing a fabrication layout generated by steps comprising: providing afirst placement for a set of the plurality of critical features, the setincluding less than all of the plurality of critical features, the firstplacement based on a design layout to implement the device; providing asecond placement for the set of the plurality of critical features, thesecond placement based on a modification of the design layout, whereinthe modification moves at least one critical feature to a new location;placing a plurality of shifters with relative phases to form the set ofthe plurality of critical features, wherein shifter placement conformswith a set of shifter design rules, wherein the second placement isprovided to resolve a phase conflict in shifter placement using thefirst placement; assigning absolute phases for the plurality of shiftersbased on the second placement; and generating the fabrication layout forthe device, the fabrication layout including the plurality of shiftersand the absolute phases.
 2. The device of claim 1, wherein thefabrication layout is used for a lithographic mask.
 3. The device ofclaim 1, wherein the device is an integrated circuit.
 4. The device ofclaim 1, wherein a number of shifters based on the second placement isgreater than a number of shifters based on the first placement.
 5. Thedevice of claim 1, wherein providing the second placement creates a newnon-critical feature in the design layout.
 6. The device of claim 1,wherein the first placement is governed by a first set of feature rulesand the second placement is governed by a second set of feature rules.7. The device of claim 1, wherein the at least one critical feature thatmoved is within a predetermined distance of the phase conflict.
 8. Thedevice of claim 1, wherein the at least one critical feature that movedwas within a graphical loop including the phase conflict.
 9. The deviceof claim 1, wherein the at least one critical feature that moved is in adifferent hierarchical subunit than the phase conflict.
 10. A devicehaving a plurality of critical features formed using shifters, theshifters providing phase shifting, the device produced using afabrication layout generated by steps comprising: providing a firstplacement for a set of the plurality of critical features, the setincluding less than all of the plurality of critical features, the firstplacement based on a design layout to implement the device; placing aplurality of shifters with relative phases to form the set of theplurality of critical features, wherein shifter placement conforms witha set of shifter design rules, wherein placing the plurality of shiftersusing the first placement results a phase conflict; converting at leastone critical feature of the set of the plurality of critical featuresinto a non-critical feature to resolve the phase conflict, the at leastone critical feature being outside a phase conflict area, the convertingthereby generating a modified design layout; assigning absolute phasesfor the plurality of shifters based on the converting; and generating afabrication layout for the device, the fabrication layout including theplurality of shifters and the absolute phases.
 11. The device of claim10, wherein the fabrication layout is used for a lithographic mask. 12.The device of claim 10, wherein the device is an integrated circuit. 13.The device of claim 10, wherein the at least one critical feature thatwas converted was within a graphical loop including the phase conflict.14. The device of claim 10, wherein the at least one critical featurethat was converted is in a different hierarchical subunit than the phaseconflict.
 15. A method for providing a layout for shifters, the methodcomprising: establishing placement of a plurality of pairs of shiftersfor a set of critical features in a same hierarchical unit, wherein acritical feature employs phase shifting and the set of critical featuresconstitutes a subset of all critical features in a layout; afterestablishing placement of the plurality of pairs of shifters, performingdesign rule checking and correction on the pairs of shifters associatedwith the set of critical features; and after performing the design rulechecking and correction, assigning phase information for shiftersassociated with the set of critical features.
 16. The method of claim15, wherein the phase information comprises relative phase informationfor a pair of shifters associated with a critical feature of the set ofcritical features.
 17. The method of claim 15, wherein the phaseinformation comprises absolute phase information for a shifterassociated with a critical feature of the set of critical features. 18.A method for providing a layout for shifters, the method comprising:establishing placement of a plurality of pairs of shifters for a set ofcritical features in a same hierarchical unit, wherein a criticalfeature employs phase shifting and the set of critical featuresconstitutes a subset of all critical features in a layout; afterestablishing placement of the plurality of pairs of shifters, performingdesign rule checking and correction on the shifters associated with theset of critical features; after performing design rule checking andcorrection, assigning phase information for shifters associated with theset of critical features; after said assigning, determining whetherthere is a phase-shift conflict among the shifters associated with theset of critical features, which conflict cannot be resolved; and ifthere is a phase-shift conflict that cannot be resolved, passing controlto a physical design layout process to adjust a physical design layout.19. The method of claim 18, further comprising, if there is aphase-shift conflict that cannot be resolved, providing information tothe physical design layout process about the conflict.
 20. The method ofclaim 19, further comprising: selecting the set of critical featuresfrom a different hierarchical unit of the layout; and repeating thesteps of establishing placement and assigning phase information.
 21. Amethod for identifying phase shift conflicts, the method comprising:establishing placement of shifters for a set of critical features in asame hierarchical unit, wherein a critical feature employs phaseshifting and the set of critical features constitutes less than allcritical features in a layout; after establishing placement of shiftersfor the set of critical features, checking design rules for shifterplacement; and after checking design rules and prior to establishingplacement for all the shifters for all critical features in the layout,determining whether there is a phase shift conflict among a set ofshifters associated with the set of critical features.
 22. The method ofclaim 21, wherein the step of determining whether there is a phase shiftconflict includes assigning relative phases for the set of shifters. 23.The method of claim 21, wherein: said layout is a physical designlayout; placement of shifters occurs in a fabrication layout associatedwith the physical design layout; and if there is a phase-shift conflictamong the set of shifters, then attempting to resolve the phase-shiftconflict in the fabrication layout prior to establishing placement forall shifters.
 24. A method for identifying phase shift conflicts, themethod comprising: establishing placement of shifters for a set ofcritical features in a same hierarchical unit, wherein a criticalfeature employs phase shifting and the set of critical featuresconstitutes less than all critical features in a physical design layout,wherein placement of shifters occurs in a fabrication layout associatedwith the physical design layout; checking design rules for shifterplacement; after checking the design rules and prior to establishingplacement for all the shifters for all critical features in the layout,determining whether there is a phase shift conflict among a set ofshifters associated with the set of critical features; if there is aphase-shift conflict among the set of shifters, then attempting toresolve the phase-shift conflict in the fabrication layout prior toestablishing placement for all the shifters; and if the phase-shiftconflict cannot be resolved in the fabrication layout, then revising thephysical design layout prior to establishing placement for all theshifters.
 25. A computer readable medium for providing a layout forshifters, the computer readable medium carrying instructions to causeone or more processors to perform steps comprising: establishingplacement of a plurality of pairs of shifters for a set of criticalfeatures in a same hierarchical unit, wherein a critical feature employsphase shifting and the set of critical features constitutes a subset ofall critical features in a layout; after establishing placement ofshifters for the set of critical features, checking design rules forshifter placement; and after checking the design rules, assigning phaseinformation for the pairs of shifters associated with the set ofcritical features.
 26. A computer readable medium for identifying phaseshift conflicts, the computer readable medium carrying instructions tocause one or more processors to perform steps comprising: establishingplacement of shifters for a set of critical features, wherein a criticalfeature employs phase shifting and the set of critical featuresconstitutes less than all critical features in a layout; afterestablishing placement of shifters for the set of critical features,checking design rules for shifter placement; and after checking thedesign rules and prior to establishing placement for all shifters forall critical features in the layout, determining whether there is aphase shift conflict among a set of shifters associated with the set ofcritical features.